OELDs have been actively studied and developed because of their capabilities of emitting light of high brightness at a low driving voltage. An OELD generally comprises a pair of opposing electrodes having therebetween a light emitting layer or a plurality of organic layers including a light emitting layer. With an electric field applied to the opposing electrodes, electrons and positive holes are injected from the cathode and the anode, respectively, into the light emitting layer, where they are recombined to form excitons, which emit light. Otherwise, energy is transferred from the excitons to create excitons of other molecules, which emit light. The light thus emitted from the light emitting layer is utilized to display an image.
For the purpose of improving driving durability of OELDs, it has been proposed to use a metal complex as a host material in the light emitting layer as disclosed, e.g., in JP-A-2002-305083. There still has been room for further improvement on efficiency and durability.